PROJECT SUMMARY Medical management of coronary artery disease patients is most commonly achieved through dual antiplatelet therapy with aspirin and clopidogrel in order to reduce rates of recurrent atherothrombotic events. While clopidogrel is generally effective, substantial inter-individual variation in platelet response to this medication has been documented and patients who have altered clopidogrel response have increased risk of experiencing a cardiovascular event and would likely benefit from alternative treatment strategies. Previous investigations have shown that clopidogrel response is highly heritable; however, apart from CYP2C19*2, identification of genetic factors that are reproducibly associated with clopidogrel response has been limited. We have previously shown that a missense loss-of-function single nucleotide polymorphism (SNP) in exon 4 of carboxylesterase 1 (CES1) results in a catalytic site glycine (G)-to-glutamic acid (E) amino acid substitution at position 143 (G143E) and substantially impacts response to clopidogrel. Importantly, CES1 is the primary enzyme responsible for metabolizing the clopidogrel prodrug, its intermediate metabolite (2-oxo-clopidogrel), and the final bioactive thiol metabolite into biologically inactive carboxylic acid derivatives. Therefore, genetic variation affecting CES1 expression and/or activity is likely to be a critical determinant of clopidogrel efficacy. However, no investigation to date has characterized the impact of genetic variation in CES1 on clopidogrel response. In this proposal, our overall hypothesis is that comprehensive characterization of CES1 will unveil novel variants that significantly impact variable clopidogrel response and that use of an alternative P2Y12 receptor inhibitor (i.e. ticagrelor) will reverse the effect of these variants on on-treatment platelet function. We will test this hypothesis by leveraging existing exome and whole genome sequencing data in 5,000 individuals to bioinformatically prioritize genetic variation in CES1 and then assess the impact of these variants on clopidogrel efficacy in participants of the Pharmacogenomics of Anti-Platelet Intervention Study (N = 566) and International Clopidogrel Pharmacogenomics Consortium (N = 5,819). We will extend these findings by performing a prospective, randomized crossover study of clopidogrel (75 mg per day for 7 days) and ticagrelor (90 mg twice daily for 7 days) in healthy individuals by CES1 genotype (G143E and the most significantly associated variant identified in Specific Aim 1, 30 individuals per genotype group) in order to assess the interaction of genotype and drug choice on on-treatment agonist-stimulated platelet aggregation. These studies will contribute to our knowledge regarding the genetic underpinnings underlying clopidogrel resistance and will assess the impact of alternative antiplatelet therapy in individuals who are genetically predisposed to altered clopidogrel response. Understanding drug response variability and the development of novel treatment strategies is critical to enhance personalized medicine initiatives, optimize cardiovascular pharmacotherapy, and ultimately reduce adverse patient outcomes.